This invention relates to an ink jet head for use in an ink jet printer and, more particularly, to an ink jet head designed to hinder formation of an air bubble or air cell in an ink tank of the ink jet head.
A conventional ink jet head of this kind will be described below with reference to FIGS. 5 to 8.
Referring to FIGS. 5 to 8, the ink jet head has a flat nozzle block 1 having a flat rectangular shape. The nozzle block 1 is formed by superposing a glass base plate 2A and a vibrating plate 2B on each other. A plurality of flow passages 3 each having an end formed into the shape of a nozzle 4 are formed in the inner surface of the base plate 2A having a substantially large thickness by etching the base plate 2A, as shown in FIGS. 6 and 8 in detail. On the upstream side of each ink flow passage 3, a constriction section 5 is formed which serves to check a back flow of ink when the ink is jetted through the nozzle 4.
A common electrode 6 is formed on a surface of the vibrating plate 2B at a position corresponding to the flow passages 3 between the nozzles 4 and the constriction sections 5. A plurality of piezoelectric elements 7 are formed on the surface of the common electrode 6 at positions corresponding to the flow passages 3, and selection electrodes are lead out from the surfaces of the piezoelectric elements 7. The piezoelectric elements 7 can be selectively energized in accordance with printing information to vibrate the corresponding portions of the vibrating plate 2B so that the ink in the corresponding ones of the flow passage 3 is jetted through the nozzles 4.
A tank member 8 is connected to a base end of the flat nozzle block 1. The tank member 8 has an overall rectangular parallelepiped shape, and a cylindrical projection 10 is integrally formed on an end surface 9A of the tank member 8 generally at the center thereof so as to project outward. An ink tank 11 is formed in the tank member 8. The ink tank 11 extends from the projection 10 on the end surface 9A to another end surface 9B opposite to the end surface 9A. The ink tank 11 has a small diameter at the end surface 9A and is extended in the longitudinal direction of the tank member 8 at an intermediate position between the two surfaces so that its longitudinal size at the end surface 9B corresponds to about 3/4 of that of the end surface 9B. The base end of the nozzle block 1 is fitted into the tank member 8 at the end surface 9B so that the ink passages 3 communicate with the ink tank 11. An O ring 12 is fitted around an outer peripheral portion of the nozzle block 1 in the tank member 8 to prevent the ink from leaking along the outer peripheral surface of the nozzle block 1. The projection 10 of the ink tank 11 is connected to an ink supply source (not shown).
In this arrangement, ink is charged in the flow passages 3 of the nozzle block 1 by being supplied from the ink supply source through the ink tank 11 of the tank member 8, and the piezoelectric elements 7 are selectively energized in accordance with printing information to jet the ink through the nozzles 4 of the corresponding flow passages 3 so that color dots are formed on a printing sheet (not shown). It is possible to effect printing on the printing sheet by repeating such selective ink injection from the nozzles 4.
In the above-described conventional ink jet head, however, no air bleeder for discharging air from the ink tank 11 is formed and the ink is directly introduced into the ink tank 11 from the ink supply source. Air contained in the ink is separated from the ink and moves to an upper section of the ink tank 11 to form a bubble or air cell. The volume of the air cell in the upper section of the ink tank 11 is gradually increased as printing operation is continued. There is a possibility that the air cell finally reaches at least a part of the port of the uppermost flow passage 3 so that the amount of ink introduced into the uppermost flow passage 3 is reduced or, in the worst case, no ink is supplied to this flow passage 3. Thus, there is a risk of instability of ink jetting from the nozzle 4 of the uppermost flow passage 3 and, hence, a deterioration in printing quality.
A conventional means for avoiding such a problem is known which resides in previously filling the ink tank 11 of the tank member 8 with ink at the time of assembly of the nozzle block 1 and the tank member 8. If the nozzle block 1 and the tank member 8 are assembled in this condition, there is a risk of the ink in the ink tank 11 attaching to portions of the ink jet head, and the assembly is therefore difficult. On the other hand, since no special means for removing bubbles contained in the ink is provided, the possibility of a substantially large air cell being formed in the upper section of the ink tank 11 during continued printing is still high and, in some case, printing qualities are considerably influenced by such an air cell.